Modern mobile communication systems make use of several communication standards known as the Universal Mobile Telecommunication System (UMTS) and Long Term Evolution, LTE. The third Generation Partnership Program, 3GPP work on LTE is also referred to as Evolved Universal Terrestrial Access Network (E-UTRAN). LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink, and is thought of as a next generation mobile communication system relative to UMTS. In order to support high data rates, LTE allows for a system bandwidth of 20 MHz, or up to 100 MHz when carrier aggregation is employed using OFDM as modulation type. LTE is also able to operate in different frequency bands and can operate in at least Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes.
Hybrid Automatic Repeat reQuest (HARQ) is an integral part of the 3G and 4G standards that allows reliable communication between a wireless device and a network node by means of incremental redundancy. The transport block to be transmitted is subjected to forward error correction encoding by which redundancy is introduced. The number of bits increases due to the introduced redundancy, but not all bits are sent at the same time. The resulting bits are segmented into several redundancy versions, where each such redundancy version comprises the same set of information bits, but different sets of parity bits. The redundancy versions are further punctured before being sent in order to fit it within the given allocation (one or more resource block pairs). “Puncturing” in this regard is a term used in coding theory, in which some of the parity bits are removed after encoding with an error-correction code. How much is punctured is depending on how many bits (information plus redundant bits) that can be carried in the allocation, which further is depending on the allocation bandwidth, the modulation (e.g. QPSK, 16QAM, 256QAM) in use, and the presence of broadcasted signals and channels in the allocated bandwidth. The ratio between the information bits and information bits plus redundant bits in a transport block is referred to as code rate. In good radio conditions, the code rate can be close to 1 (very little redundancy), and it decreases with worsened radio conditions (gradually increasing redundancy of information). The combination of code rate and modulation type is referred to as Modulation and Coding Scheme, or short MCS. MCS and other parameters affecting the transmission robustness are referred to transmission properties.
In practice, incorrectly received coded data blocks are often stored at the receiver rather than discarded, and when the re-transmitted block is received, the two blocks are combined. While it is possible that two given transmissions cannot be independently decoded without error, it may happen that the combination of the previously erroneously received transmissions gives us enough information to correctly decode. This approach is called Hybrid ARQ with soft combining, in which incremental redundancy is a possibility for such soft combination.
Incremental redundancy allows the receiving node, for example a user device, to attempt to receive and decode a first redundancy version of the transport block. In case it fails, it receives a second redundancy version of the transport block, which is combined with the first received block and again decoded. In case a re-transmission of the same transport block is required, such occur normally at minimum of 8 ms distance. Under some circumstances, features such as TTI bundling are used, by which several redundancy versions are transmitted in subsequent subframes without waiting for feedback on whether a previous redundancy version was successfully decoded.
The receiving entity provides acknowledgment to the transmitting entity on whether it decoded the transport block successfully (ACK) or whether it failed (NACK). The transmitting entity then can decide on whether to transmit another redundancy version for the same block, or send a redundancy version for a next transport block.
In case the maximum number of retransmissions is reached without the receiving entity being able to decode the transport block, it will be detected by higher layers e.g. Radio Link Control, RLC, generally within 50-100 ms that a Protocol Data Unit (PDU) is missing and a retransmission is requested for all transport blocks that comprise the RLC PDU, even those that may have been successfully decoded. This is referred to as Automatic Repeat request (ARQ) and has considerably larger latency than HARQ retransmissions.
In a LTE network, a wireless device (in LTE referred to as a User Equipment, UE) carries out measurements to provide indications to the base station (in LTE eNodeB) on the perceived radio propagation conditions in what is called Channel Quality Indicator (CQI) reporting. Based on the reporting, the base station can roughly decide on the Modulation and Coding Scheme to use for communication with the UE. An example of mapping between CQI and MCS is shown in Table 1 below, retrieved from 3GPP TS 36.213 V10.12.0 section 7.2.3. In low channel quality (low CQI index) more forward error correction encoding is needed for successful decoding of the information bits, and vice versa in high channel quality i.e. high CQI index. Hence, at high CQI the throughput of information bits can be made higher than at low CQI.
TABLE 14-bit CQI table from 3GPPCQI indexmodulationcode rate × 1024efficiency0out of range1QPSK780.15232QPSK1200.23443QPSK1930.37704QPSK3080.60165QPSK4490.87706QPSK6021.1758716QAM3781.4766816QAM4901.9141916QAM6162.40631064QAM4662.73051164QAM5673.32231264QAM6663.90231364QAM7724.52341464QAM8735.11521564QAM9485.5547
In order to get a good system throughput, the eNodeB carries out link adaptation matching each UE's reported channel quality to an MCS that provides the right balance between system throughput and throughput for the individual user. The MCS is indicated to the UE in the Downlink Control Information, DCI provided over Physical Data Control Channel, PDCCH as shown in Table 2 retrieved from 3GPP TS 36.213 V10.12.0 section 7.1.7.1.
TABLE 2Modulation and TBS index table for PDSCHMCS IndexModulation OrderTBS IndexIMCSQmITBS0201212223234245256267278289291049114101241113412144131541416415176151861619617206182161922620236212462225623266242762528626292reserved304316
In addition to CQI reporting, eNodeB typically has an outer loop that tunes the MCS value based on ACK/NACK reports to a suitable value giving a BLER (ratio between NACKs and total number of received or expected ACK/NACKs) of e.g. 10%. Besides catering for flexibility in which target BLER is used (e.g. 1%, 10%, 30%), it also solves the problem that each UE model or even UEs of the same model may have an individual bias in the reported CQI. The base station thus maintains a UE-specific CQI offset which it tunes to give the desired BLER target.
In case of UEs with extreme requirements on reliable low-latency communication, e.g. residual BLER in the order of 10−9 and latency in the order of 20 ms (i.e., at most one transport block per billion is allowed to have latency exceeding 20 ms), block errors on MAC level have to be avoided as far as possible. Each block error increases the risk of a failure in the MAC HARQ combining, resulting in a RLC retransmission (RLC ARQ), typically associated with a latency of 100 ms or more. This is due to that RLC has to wait for some time after it has detected that a package has been delivered out-of-order (configurable by the network operator) before it can conclude that a MAC PDU has been lost and can request a retransmission.
The existing HARQ implementation only supports binary reporting on whether a transport block has been correctly decoded, i.e. ACK or NACK. This means that the operation of determining and compensating for the unique CQI bias cannot be done without increasing the risk for introducing block errors on MAC HARQ level that may lead to increased latency since each unsuccessful HARQ retransmission takes at least an additional 8 ms. The reason is that when a block is correctly decoded, the positive acknowledgment ACK will not tell the sender, how close to its limit the decoding operation was, i.e. if there were correctable error or not.
The existing implementation makes it hard for the network node to identify the CQI bias, because such operation may lead to violation of latency requirements and/or reliability requirements. As a result, eNodeB needs to configure DL and UL transmissions using a much more robust MCS than called for, in order not to risk introducing delays and/or block errors on RLC level. More robust MCS leads to a higher usage of resources for the particular UE than necessary, with fewer resources available for other UEs in the cell, thereby reducing the overall system throughput.